Abstract
Alterations in the lipid composition of lipid rafts have been demonstrated both in human brain and transgenic mouse models, and it has been postulated that aberrant lipid composition in lipid rafts is partly responsible for neuronal degeneration. In order to assess the impact of lipid changes on lipid raft functional properties, we have aimed at determining relevant physicochemical modifications in lipid rafts purified from frontal cortex of wild type (WT) and APP/PS1 double transgenic mice. By means of steady-state fluorescence anisotropy analyses using two lipid soluble fluorescent probes, TMA-DPH (1-[(4-trimethyl-amino)phenyl]-6-phenyl-1,3,5-hexatriene) and DPH (1,6-diphenyl-1,3,5-hexatriene), we demonstrate that cortical lipid rafts from WT and APP/PS1 animals exhibit different biophysical behaviors, depending on genotype but also on age. Thus, aged APP/PS1 animals exhibited slightly more liquid-ordered lipid rafts than WT counterparts. Membrane microviscosity ηapp analyses demonstrate that WT lipid rafts are more fluid than APP/PS1 animals of similar age, both at the aqueous interface and hydrophobic core of the membrane. ηapp in APP/PS1 animals was higher for DPH than for TMA-DPH under similar experimental conditions, indicating that the internal core of the membrane is more viscous than the raft membrane at the aqueous interface. The most dramatic changes in biophysical properties of lipid rafts were observed when membrane cholesterol was depleted with methyl-β-cyclodextrin. Overall, our results indicate that APP/PS1 genotype strongly affects physicochemical properties of lipid raft. Such alterations appear not to be homogeneous across the raft membrane axis, but rather are more prominent at the membrane plane. These changes correlate with aberrant proportions of sphingomyelin, cholesterol, and saturated fatty acids, as well as polyunsaturated fatty acids, measured in lipid rafts from frontal cortex in this familial model of Alzheimer's Disease.
Highlights
The existence of membrane rafts was disputed for many years, there is a general agreement that these specialized membrane nanodomains are specialized signaling platforms involved in a number of physiological functions under normal and pathological conditions (Allen et al, 2007; Michel and Bakovic, 2007)
We present the results for each probe separately and, at the end of this section, we described the results of microviscosity-lipid relationships
TMA-DPH Results summarized in Figure 2A show the TMA-DPH anisotropy values measured in the range 20–40◦C in lipid rafts from wild-type and amyloid precursor protein (APP)/PS1 animals aged to 6 and 14 months
Summary
The existence of membrane rafts was disputed for many years, there is a general agreement that these specialized membrane nanodomains are specialized signaling platforms involved in a number of physiological functions under normal and pathological conditions (Allen et al, 2007; Michel and Bakovic, 2007). Recent evidence has demonstrated that alteration of lipid raft homeostasis is linked to deregulation leading to neuronal loss in Alzheimer’s disease. Alterations in these microdomains are known to affect amyloid precursor protein (APP) processing and neurotransmitter signaling (Rushworth and Hooper, 2006; Williamson and Sutherland, 2011; Hicks et al, 2012). Lipid rafts play a central role in proteolytic processing and regulation of APP cleavage, and recent reports have shown that lipid rafts are the subcellular sites of amyloidogenic β-amyloid production by β-secretase 1 (BACE1) and the γ-secretase complex (Rushworth and Hooper, 2006; Williamson and Sutherland, 2011; Hicks et al, 2012). Regression analyses were performed by non-linear regression using Sigmaplot software (Jandel Scientific, San Rafael, CA)
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